Preparation and structure elucidation of multifunctional porous TiO2 surfaces by means of plasma electrolytic oxidation

Abstract

Plasma electrolytic oxidation (PEO) is an established electrochemical process to produce of stable, compact, ceramic-like and porous oxide layers and was increasingly used during the 20th century for the structuring of metal surfaces such as aluminum, magnesium and titanium. On an industrial scale, especially in the fields of thermal protection and corrosion protection, the applications of the process have increased significantly in recent years, such as its use for decorative aspects. Likewise, PEO has become interesting for medical technology and implantology due to the diversity of its varying surface properties such as porosity, layer thickness and surface composition. The PEO-process makes it possible, especially on titanium surfaces, to produce crystalline oxides through the formation of high-energy plasma discharges. These titanium dioxides can in turn enable the photocatalytic activity of the surfaces and significantly increase their wear resistance. However, the ongoing physical and chemical processes are very complex and for some metals, as here for titanium, are not fully understood. One of the most important aspects to understanding the involved process events is the investigation into used parameters, such as electrolyte composition, applied voltage, and the resulting structure of the oxide layer. These influencing factors were analyzed in the present work, and both the structures and the surface properties were examined in detail. In this work, pure titanium materials were treated with various electrolytic systems to enable a decoding of differences in structure and the resulting properties. Distinct differences between the oxide layers in terms of phase formation and structure could be demonstrated by x-ray diffraction and microscopic examination. By increasing the voltage and choosing the right electrolyte, the structure of the oxide layers could be varied with regard to pore size and distribution as well as in layer thickness and the degree of crystallinity of the titanium dioxide phases. Using this method, the proportion of crystalline oxides in the PEO-layers could be adjusted through the right electrolyte composition and an increase in the applied voltage. By determining the surface properties and compositions of the oxide layers, it was possible to investigate these in more depth regarding their structural design, thus gaining a better understanding of the effect of the plasma discharges. Using Raman spectroscopy and the EBSD technique, the crystalline constituents could be detected and identified within the entire oxide layer. Analogical to the XRD measurements, an increase in crystalline TiO2 phases was found from the lower part of the oxide layer to the surface of the layer. Furthermore, the structure of the PEO oxide layers could be decrypted because of a detailed analysis using the STEM method, whereby large crystals in the upper area of the oxide layer and smaller crystals at the boundary layer to the titanium substrate could be visualized. For the first case, these areas were created by the high energies of the discharges in the later course of the oxidation, whereas the smaller crystals at the lower part of the oxide layer could be explained by the effects of the discharges as far as the bottom of the oxide layer. Amorphous TiO2 was detected around the generated pore structures of the oxide layer. These amorphous regions led to the conclusion that the resulting TiO2 can be converted into the liquid and gaseous phases during the process. The reduced conductivity in the gaseous phase and the surrounding colder electrolyte led to a faster cooling of the TiO2 in the area of the pore structures and thus to a reduced formation of crystalline structures. The results presented in this work demonstrate the possibility of adapting the surface properties, such as morphology, crystallinity, and photocatalytic activity of PEO oxidized titanium dioxide layers for a variety of applications. The crystallinity of the titanium dioxides can be selectively controlled and helps to adjust the stability as well as the photocatalytic activity of the layers. The transfer of thin PEO-layers to polymeric substrates, as well as the improvement of the adhesion of titanium to polymeric substrates with a special adhesive layer, has thus been successfully achieved. In addition to titanium, polymer substrates are also used as an implant material in medicine, but they often cannot withstand the biocompatibility requirements. The improvement of cell adhesion to pure polymers by applying a PEO-layer was successfully achieved. Further intensive investigations into the structure of the PEO oxide layers led to a better understanding of the process and the effects of the plasma species on the entire layer. This helped to expand the model of plasma electrolytic oxidation on titanium materials.